A superconducting material exhibits no electrical resistance when cooled below its characteristic critical temperature. Although high-temperature superconductor materials, which have critical temperatures higher than the 77K boiling point of nitrogen, have been identified, these materials are often exotic (e.g., perovskite ceramics), difficult to process, and unsuitable for high-field applications. Thus, for practical superconducting applications requiring wires and coils and bundles thereof, the metallic superconductors Nb—Ti and Nb3Sn are most often utilized. While these materials have critical temperatures below 77K, the relative ease of processing these materials (e.g., drawing into wires), as well as their ability to operate at high currents and high magnetic fields, have resulted in their widespread use.
Typical metallic superconducting wires feature multiple strands (or “filaments”) of the superconducting phase embedded within a copper (Cu) conductive matrix. While Nb—Ti is sufficiently ductile to be drawn down into thin wires directly, its applicability is typically limited to applications featuring magnetic fields having strengths below approximately 8 Tesla. Nb3Sn is a brittle intermetallic phase that cannot withstand wire-drawing deformation, and thus it is typically formed after wire drawing via diffusion heat treatment. Nb3Sn superconducting materials may typically be used in applications featuring magnetic fields having strengths up to at least 20 Tesla. Thus, several different techniques have been utilized to fabricate Nb3Sn-based superconducting wires. For example, in the “bronze process,” a large composite is fabricated from Nb rods and Cu—Sn alloy rods (that include, e.g., 13-15% Sn) surrounding the Nb rods. Since these materials are ductile, the composite may be drawn down to a suitable diameter, and then the drawn-down composite is annealed. The heat treatment results in interdiffusion and the formation of the Nb3Sn phase at the interface between the Nb and the Cu—Sn. Other processes for forming Nb3Sn-based superconducting wires similarly involve formation of the brittle Nb3Sn phase after wire drawing. For example, pure Sn or Sn alloys with Cu or Mg may be incorporated in the interior of the initial composite and annealed after drawing. Alternatively, Nb filaments may be embedded within a Cu matrix and drawn down into wire. The resulting wire may subsequently be coated with Sn. The coated wire is heated, forming a Sn—Cu phase that eventually reacts with the Nb filaments to form the Nb3Sn phase.
While the techniques detailed above have resulted in the successful fabrication of metallic superconducting wires utilized for a host of different applications, the resulting wires often exhibit insufficient electrical performance. Typical superconducting wires contain many of the Nb3Sn or Nb—Ti filaments described above embedded within, disposed around, and/or surrounded by a Cu stabilizer that provides the wires with sufficient ductility for handling and incorporation within industrial systems. Although this Cu stabilizer is not itself superconducting, the high electrical conductivity of Cu can enable satisfactory overall electrical performance of the wire. Unfortunately, various elements from the superconducting filaments (e.g., Sn) may react with portions of the Cu stabilizer, forming low-conductivity phases that negatively impact the overall conductivity of the entire wire. While diffusion barriers have been utilized to shield the Cu stabilizer from the superconducting filaments, these barriers tend to have non-uniform cross-sectional areas and may even locally rupture due to non-uniform deformation during co-processing of the diffusion barrier and the Cu stabilizer. While such diffusion barriers could simply be made thicker, such solutions impact the overall conductivity of the wire due to the lower electrical conductivity of the diffusion barrier material itself. For example, for cutting-edge and future applications such as new particle accelerators and colliders, magnets are being designed beyond existing wire capabilities; such wires will require a non-copper critical current density of more than 2000 A/mm2 at 15 Tesla. As the diffusion barrier is part of the non-copper fraction, minimizing the volume of any barrier material is important while any strength benefit is advantageous.
In view of the foregoing, there is a need for improved diffusion barriers for metallic superconducting wires that substantially prevent deleterious reactions involving the Cu stabilizer while remaining uniformly thin so as not to occupy a significant amount of the overall cross-sectional area of the wire.